The transmission of digital information and data between systems has become an essential part of commonly used systems. With such systems, information content is transmitted and received in digital form as opposed to analog form. Information long associated with analog transmission techniques, for example, television, telephone, music, and other forms of audio and video, are now being transmitted and received digitally. The digital form of the information allows signal processing techniques not practical with analog signals. In most applications, the user has no perception of the digital nature of the information being received.
Traditional modes of communication often occurred in "real time". For example, a telephone conversation occurred in real time. A "live" video conference occurred in real time. Users have come to expect these, and other such traditional forms of communication to be real time. Thus, digital transmission and reception techniques and systems need to provide for the real time transmission and reception of information.
There is a problem, however, in that digital communication between devices distant from each other usually precludes the availability of identical sampling frequencies. Except for those cases where a distinct clocking hierarchy structure can be defined and a common distributed clock source employed, there will be some difference between the sample rate of one device (e.g., the transmitter) from the other (e.g., the receiver).
Prior Art FIG. 1 shows a typical prior art digital information transmission and reception system 100. In system 100, a signal source 101, for example, a voice transmitter for a telephone, generates an analog input signal. The input signal is coupled to a sampler-ADC (analog to digital converter) 102, where it is sampled and encoded into a digital signal. The digital signal is comprised of a series of samples, representative of the analog input signal. This digital signal is transmitted across a transmission link to a FIFO (first in first out) buffer 103. Buffer 103 is coupled to a receiver-DAC (digital to analog converter) reconstruction filter 104 (hereafter receiver 104). As the samples comprising the digital signal arrive at buffer 103, they are stored and subsequently output to receiver 104 on a first in first out basis. The "stream" of samples are subsequently coupled to the DAC-reconstruction filter included within receiver 104, where they are decoded and filtered into an analog output signal, for example, the user's voice. The output signal represents the input signal from signal source 101.
In system 100, as in other typical prior art digital transmission systems, the transmitter (e.g., sampler-ADC 102) and receiver 104 operate at different sampling clock frequencies. Additionally, the ratio between the sampling clock frequency of sampler-ADC 102 and the sampling clock frequency of receiver 104 usually is a non-integer value. Hence, even though information is being transmitted at the same nominal sampling frequency, when the local clocks are not the same, there will usually be a slight difference in the actual sampling rates. As a result, sampling at the sending terminal (e.g., sampler-ADC 102) and reconstruction at the receiving terminal (e.g., receiver 104) is accomplished with a slight variance from the nominal sampling rates.
In addition, the sample rate frequencies may also vary over temperature, part scattering, and time. For clock ratios with a fractional part, a sample rate exists at which sample overruns or underruns will occur in buffer 103 due to its fixed capacity. Those overruns and underruns are hereafter referred to as sample slippage. Sample slippage generates objectionable distortion, for example, in the form of an audible click noise in audio transmissions or telephony applications, and horizontal jitter in television systems. In some systems, the error due to slippage is cumulative and segments of transmitted information are backed-up and/or delayed. Over a period of time, such segments may eventually be lost, especially if the system is designed to re-synchronize, or attempt to re-synchronize, itself to a real time or master clock.
For example, where the sample rate of the receiver is slower than that of the transmitter, the many samples comprising the received digital signal eventually overrun buffer 103, forcing the receiver 104 to "throw away" an amount of samples in an attempt to re-synchronize itself. Where the sample rate of the receiver is faster than that of the transmitter, the samples comprising the received digital signal cause an underrun condition at buffer 103, forcing the receiver 104 temporarily pause operation in order to re-synchronize itself. In either case, the resulting output signal is distorted and noisy with respect to the input signal.
Thus, what is required is a system for digital transmission which overcomes sample slippage limitations. The required system should provide for digital transmission and reception systems which eliminate sample slippage distortions. Additionally, the required system should function transparently with respect to users of the system. The system of the present invention should not cause noticeable distortion as it operates to remove sample slippage. The present invention provides a novel solution to the above requirements.